Ladder-type filters are used in high frequency communication systems. It has been known to connect a resonant circuit including an inductor and a capacitor connected in parallel between the parallel resonator of a ladder-type filter and ground as disclosed in Japanese Patent Application Publication Nos. 9-261002, 9-321573, and 2008-205947 (Patent Documents 1 through 3). It has been also known to interconnect the ground terminals of two or more parallel resonators by a capacitor or the like as disclosed in Japanese Patent Application Publication Nos. 2004-72411 and 2012-175438 (Patent Documents 4 and 5).
The provision of the resonant circuit disclosed in Patent Documents 1 through 3 allows an attenuation pole to be formed outside the passband of the ladder-type filter. However, the resonant circuit forms a single attenuation pole. Therefore, two or more attenuation poles are not formed.